Do filenames with asian characters, smileys or other special characters look like this in your JDownloader?

... then your used font cannot render unicode points!
You will have to chose another font:

1. Got to Settings -> Advanced Settings -> Search for:
   LAFSettings: Font Name
2. Enter e.g. Arial Unicode MS or Yu Gothic UI in the "Value" field and click somewhere in the empty space to confirm.
3. Restart JD and these brackets should be gone: